Members of the family Flaviviridae are important human pathogens and fall within the NIAID class A, B and C priorities, depending on the virus species. Like many other viruses, flaviviruses have evolved complex mechanisms to interact with the host immune response. The earliest immune responses to infection involve components of the innate immune system, starting with the recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs), such as toll-like receptors (TLRs), and intracellular RNA helicases, such as RIG-I and Mda-5. PAMP recognition results in activation of specific signal transduction pathways that result in production of proinflammatory cytokines, such as type I interferon (IFN), TNFa and others. Type I interferon is one of the most important mediators of the early immune response. Mice deficient in IFN signaling are more susceptible to flaviviruses and pretreatment of mice with type I IFN can prevent flavivirus disease. One of the major pathways of IFN synthesis in viral infection is the stimulation of plasmacytoid dendritic cells (pDCs) via TLRs 7, 8 or 9. While several studies have indicated that flaviviruses are able to stimulate pDCs to produce IFNa, the mechanisms of this stimulation are not understood. Our preliminary data indicates that mosquito-derived flaviviruses are unable to stimulate TLR7 while viruses grown in mammalian cells strongly stimulate TLR7. We hypothesize that differential glycosylation of viruses grown in either mammalian or insect cells accounts for theses differences. In addition, our previous work has shown that WNV, and more specifically its NS1 protein, is able to interfere with TLRS and TLR7 signal transduction which could prevent further activation of these pathways after initial nfection has been established. TLR7 is a PRR that recognizes ssRNA and activates the transcription factors NFkB and interferon-regulatory factor 7 (IRF7), resulting in transcription of IFNa and other cytokines. NS1 is glycoprotein secreted to high levels during flavivirus infection that can be endocytosed by several cell types. We hypothesize that secreted NS1 is able to inhibit TLR7 function and thus contributes to viral pathogenesis. We have developed three specific Aims to address our hypotheses. In Aim 1 we will identify mechanisms of flavivirus recognition by TLR7 with a specific focus on differential glycosylation of viruses grown in either mammalian or insect cells. In Aim 2 we will determine mechanisms of TLR7 inhibition by the lavivirus NS1 proteins, focussing on a potential role for secreted NS in this inhibition. In Aim 3 we will nvestigate the contribution of TLR7 signaling to WNV pathogenesis.